This invention relates to compression release brakes and, in particular, to apparatuses for adjusting timing of such brakes during brake operation by varying hydraulic pressure acting on the slave piston.
Compression release brakes are well known and are typically employed on large diesel engines used to power vehicles. In theory the compression of a charge in an engine should slow the vehicle when the fuel supply to the engine is cut off so as to reduce reliance upon the wheel brakes. However there is a rebound effect after the charge in each cylinder has been compressed by the piston. The compressed gases in each cylinder after the compression stroke tend to act against the piston and drive it away from the cylinder head, effectively counteracting the braking effect just described.
It was realized in the past that this rebound effect could be removed by opening each exhaust valve at a time when the valve would normally be closed, namely when the piston is near the top dead center position of the compression stroke. The compressed gases therefore are released through the exhaust valve into the exhaust manifold, removing the rebound effect so that the compression of gases in the cylinder in fact achieves the desired braking effect.
The most common type of engine brake utilizes a master hydraulic cylinder connected to a slave hydraulic cylinder by a hydraulic system. The master cylinder has a master piston which operatively contacts some component of the engine which reciprocates the piston of the master cylinder at the desired time near the top dead center position for each cylinder of the engine. The slave cylinder has a slave piston which operatively engages the exhaust valve opening mechanism. Accordingly, when the master piston is reciprocated, the slave piston bears against the exhaust valve opening mechanism, opening the exhaust valve near the top dead center position on the compression stroke.
These brakes are usually controlled by a switch mounted in the cab of the vehicle connected to a solenoid valve. The solenoid valve provides hydraulic fluid to the engine brake to render it operational.
Time is critical for optimal operation of such engine brakes. If the exhaust valve is opened too early, then the braking force is not maximized since compression is lost before the air in the cylinder is completely compressed. However, if the timing is left too late, then it may be difficult to open the exhaust valve against the force of the compressed gases in the cylinder. This may place too heavy a load on the engine. Alternatively, the compressed gases may not be released on time to avoid some rebound effect. Timing which may be ideal or acceptable at low engine speeds may be too late at higher engine speeds, leading to unacceptably high loading as discussed above.
Typically such timing problems are dealt with by the amount of gap or lash between the slave piston and the exhaust valve opening mechanism. A number of previous patents have provided different approaches to adjusting the lash and accordingly the timing. For example, my own earlier U.S. Pat. No. 4,655,178 and 4,898,128 revealed devices which can take up lash during brake operation in whole or in part. It is even possible to achieve a state of negative lash wherein the valves are held open to some degree throughout operation of the brake.
However it is desirable in some circumstances to provide lash adjustment on an ongoing basis during operation of the engine and brake. One approach to this is found in U.S. Pat. No. 5,379,737 to Hu which discloses an electrically controlled timing adjustment for compression release brakes. In this patent a solenoid-activated member pushes on the slave piston to reduce or eliminate the gap between the slave piston and the valve operating mechanism.
Other patents outlining methods of achieving slave piston lash control include U.S. Pat. No. 4,398,510 and 4,475,500.
Most of the prior art attempts at lash control sacrifice low speed performance apart from the Hu patent discussed above. As fuel economy concerns rise, engines are being operated at lower speeds, resulting in better economy, but poor engine brake performance with conventional engine brakes.
Accordingly it is an object of the invention to provide an improved compression release brake which overcomes the deficiencies associated with the prior art devices but is simple in construction, reliable in operation and economical to produce.
It is also an object of the invention to provide an improved compression release brake which improves brake performance at lower engine speeds compared to prior art devices.
It is a further object of the invention to provide an improved compression release brake which allows the timing of the brake to be adjusted during engine operation to improve performance.
It is a still further object of the invention to provide an improved compression release brake with the capability of selecting between at least two different slave piston lash settings during operation of the brake.